If you can‘t install macOS on a Time Machine backup disk

If you try to install macOS on a Time Machine backup disk, you might see the message “Cannot install on a volume used by Time Machine for backups.”

The macOS installer is designed to prevent you from installing macOS on a Time Machine backup disk. It's a good idea to keep your startup disk and backup disk separate, in case your startup disk becomes unusable.

If you no longer want to use your Time Machine backup disk for backups, you can either erase the entire backup disk, or follow these steps to erase just the backups on the disk. You can then install macOS on the disk.